This invention is directed to the arrangement of windings and electronic switches used in inductive devices (for example, transformers and regulators) in order to construct a more economical, fully electronic on-load tap changing mechanism. Tap changing is used extensively in a wide variety of electrical inductive apparatus, such as AC voltage regulating transformers, high voltage DC (HVDC) rectifier and inverter transformers and phase angle regulators, to adjust device turns ratio or phase angle while the device is serving load.
Most of the tap changing methods in present commercial use make use of a mechanical switching means to alternately connect various sections of winding of the electrical inductive apparatus into a circuit. One extensively used switching means is a mechanical contact switch in which a movable contact, which selectively engages stationary contacts, is connected to various sections of the winding in order to connect varying numbers of turns into the circuit. This technique is at present used to the virtual exclusion of all other methods in large power apparatus. In applications where these mechanical devices are used, the arrangement of the windings is such that the buck (decrease) and boost (increase) voltage capable of being provided will be the same magnitude. For example, in the utility industry in the United States of America the most common increase/decrease is .+-.10%.
The electronic switch is another type of switching means, which has generated significant interest recently due to its fast response time and lack of mechanical wear. Because of its fast reaction time it can be used to mitigate voltage sags and swells in addition to performing the more traditional duties of an on load tap changer such as voltage leveling. Electronic switches are typically electronically controlled gate devices, such as thyristors and gate turn-off (GTO) devices, which are configured as an inverse parallel-connected pair to each tap of a winding, as shown in U.S. Pat. No. 3,195,038. Further, tapped secondary windings may be utilized with appropriate switching devices to increase the tap range of the electrical inductive apparatus as shown in U.S. Pat. Nos. 3,195,038, 3,909,697 and also 3,700,925. U.S. Pat. No. 5,604,423 teaches an electronic tap changing concept of Discrete-Cycle Modulation (DCM) whereby tap voltage magnitudes are obtained in increments intermediate to the physical tap winding voltage magnitudes. Again, however, in the winding configurations demonstrated in these patents, the buck and boost voltage of the tap changer are the same magnitude.
Today, an increasing amount of industrial and commercial equipment contain electronic components and controls which will not function properly when the voltage supply fluctuates. For example, variable frequency drives, plastic fabrication equipment, microwave heating, and computers are some of the more common loads which are sensitive to supply voltage variations.
Deviations from the nominal ideal of 100% voltage magnitude result from many causes, the most common being voltage sag. Voltage sags are a decrease in the supply voltage that may last from 1 cycle to several seconds and may decrease the supply voltage by 10% to 80% of the nominal supply voltage. The occurrences of sags is the largest and most costly power quality problem facing industrial and commercial concerns today.
Therefore, there is a need in the power utility industry for a new electronic tap changer arrangement for a regulator or transformer which comprises a commercially viable arrangement and which particularly addresses the occurrence of sags within the electrical transmission and distribution system. The present invention addresses this need by providing a regulator/transformer with unsymmetrical boost/buck characteristics.